My observations from the years I drove with a detector on the visor reflect pretty much what Than posted above. They're not fool proof. With the modern pulse type laser, by the time your detector registers a hit, your speed is locked in on the dash and your done. I doubt very much that there is enough laser reflection/wash off of a vehicle up front to give any reliable signal to you, a mile back. After mine failed, I elected not to replace it. YMMV